This project will attempt to define, then prevent, delay, or reverse alloimmunization to platelet transfusions. We will evaluate patients prior to transplantation for evidence of alloimmunization based on antibody testing and radioisotope platelet survivals. Nonalloimmunized patients will be randomized to receive routinely prepared blood products or into two study arms: platelet transfusions exclusively from single unrelated donors given sequentially or platelets and red cells depleted of leukocytes. We will compare transfusion requirements, response to platelet transfusions and the development of lymphocytotoxic and platelet antibodies in these three groups. Methods will be developed for ultraviolet irradiation (UVI) of blood products and the prevention of alloimmunization with these products will be studied. In selected patients with antibodies to platelets prior to transplant, plasma exchange will be performed to remove antibodies kinetics will be evaluated by comparison to a similar group of alloimmunized patients who are not plasma exchanged. If plasma exchange is successful, specific methods of removing antibodies by ex vivo immunoabsorption with whole platelets will be developed.